The invention relates to a speed control device of a commutator motor, and more particularly relates to a safety control of the motor in a circuit controlling the speed of the commutator motor by means of a semiconductor control element for rectifying the full waves of a power source current. According to the invention, if an extraordinary load is applied to a feed back control device provided to reduce the speed variation of motor due to the variation of load, the load current of motor is instantly stopped to prevent the motor from being overheated and burnt. With the subsequent stop designation, the motor becomes ready to be driven again, thereby to solve the problem of the safety and maintenance as to the conventional permanent stop or automatic restoration of the motor. Further in the conventional speed control circuit of the motor, the motor is instantly driven with application of the power source in a condition that the latter is blocked while the motor is designated to drive. Such a dangerous phenomenon is prevented by the invention. According to the invention, the motor, once stopped, will be started only with a stop designation applied after the control power source is applied. Thus the invention provides the abovementioned two functions in a speed control circuit of commutator motor, which are commonly controlled by an electronic control circuit.
As a conventional device of such a kind, it has been proposed for detecting an excessive load of a motor to set a certain data which is a maximum permissible value to normal performance of the motor so as to limit the conduction angle of voltage waves supplied to the motor, and which is adapted to determine the ignition position of a maximum ignition phase irrespectively of a speed designation of motor for the purpose of blocking the current flowing to the motor when the ignition phase has reached the maximum ignition phase, by means of a feedback control to compensate the variations of rotation speed of motor which may be caused by the variations of load. In this case, it is desirable that the set value is such that the ignition phase as the result of the feedback control comes to the maximum one on a condition that an excess load is applied to the motor in case a maximum speed is designated. So long as the motor is controlled in the vicinity of the set value, the rotation speed of motor is constant, and accordingly a high speed is maintained with a provision of self-cooling function preventing the current value form overheating or burning the motor. Such a predetermined maximum ignition phase is, however, not suitable to the security as to the excess load in a low speed control of the motor.
Namely, if the motor is overloaded under a low speed control, the ignition phase advances to maintain the rotation speed by way of the feedback control. If the feedback amount increases as far as the control of the maximum ignition phase, the current flowing to the motor is blocked. The motor is however maintained in a low speed rotation as long as it is controlled in the vicinity of the set value. Therefore the selfcooling function of motor is lowered compared with that in a high speed rotation. Precisely the motor speed electromotive force, i.e. the voltage functioning in the direction for blocking the motor current due to the rotation of motor, is lowered, and equivalently the inner impedance of motor is lowered. This increases the load current of the motor which is driven with the equivalent voltage as in the high speed rotation thereof due to the same ignition angle. Therefore the motor driven with a value in the vicinity of the set value is more overheated in a low speed control than in a high speed control. This shows that the set value is improper to the security of the motor. For the purpose of eliminating such a defect, it has been proposed to provide some set values in accordance with the rotation speeds and different amounts of loads for maintaining the security of the motor. Such device is however, complex in structure and costly.